If you’re not sure what C1 is doing, generating previews or not, a simple click on the rotating activity wheel thingy will open up the activity window (I’m pretty sure that it’s accessible through the window menu too but don’t have my laptop at hand). If C1 is generating previews it’ll show you that and what percentage is done.

I'm probably working with smaller files than you (older cameras) but typically a shoot of some sort will produce either about 1000 images or for a multiday shoot 2000 to 3000 images and perhaps on to larger numbers from there.

I will typically put the images into subfolders of the C1 Session created "Captures" folder (really a convenience folder for tethered studio type shoots but convenient anyway for all gathered images as it makes archiving a session very easy.)

However I have other sessions where I don't do that and simply connect to a folder as a favourite. Or don;t connect at all it is it likely to be just a one-off edit that I don't think I will need to go back to.

If you start C1 and select an existing session CI will head off and check the state of the session at last close and decide what to do next. The default decision (assuming the session closed cleanly as they should and normally do) is to automatically open it at where it was last left.

To do that C1 will identify the last 'collection' that was in use (An album/Smart Album/Folder or whatever) and go and read the files it needs from disk and load them into the working memory cache (Temporary memory used when working with the files involved).

My understanding (from something I read some time ago) is that there is some code applied that tries to prioritise efficiency in the way it creates a working index for the files. So it will open the image last in use but also try to guess what you might be doing next - in simple use perhaps pre-prepare and load the 'next' image or the 'previous' image. Of course it has to make some assumptions to be able to do that and things like the current sort order in use and the current 'group' of images may influence how that works. Also current filters and so on.

It will also set up the previews and thumbnails according to any view settings being applied - so output recipes, watermarks, exposure warning, focus mask, etc., etc.

These settings are applied to the files in active memory rather than written to disk. IF something significant for the Preview or Thumbnail files is eventually saved they may be changed later and written back to disk when you finish editing the image or when closing the session.

If you have a look in the Capture One folders associated with the session's image and check the 'Cache' folder (this is not the "In Memory" cache but just a folder named "Cache", you should see a number of files that relate to the images in the "Proxies" and "Thumbnails" and the dates that they were created/modifed or whatever the operating system shows you.

So you then work away with the session - for large sessions one might expect the working memory to be updated and refreshed from disk as one works with it - and any outstanding changes that need to be written back to disk when you close the session (and possibly when changing to different groups of images as you work) will be written back to the disk folders as part of the session shut down.

Mostly systems and software will have a limit to the amount of internal memory they can address (given enough installed memory to reach that limit).

I also have 24Gb of RAM in my PC.

The total load is currently about 14.5GB used with several programs running in addition to C1.

In C1 I have a 2,500 image session open and it is using about 7.5GB across all of its processes.

From memory I don't think I recall it using more than about 8GB or so when just editing. (Assuming that it can grab 8Mb when in competition with the operating system and other applications.) It is possible that in some situations a larger amount of memory may be used where multiple operations are being performed at the same time. Running a batch output process for example.

It's difficult to address your folder structure question.

From what you wrote the number of images actually managed in C1 should not be a great challenge for the application whether you run with a session per year approach or a catalogue.

I use sessions and they make sense to me. I don't really feel a need to open a huge database because I have a few new images to edit and if I have been shooting some sort of event keeping that set of images in a session (no matter what I do with the folder structure when importing) makes sense to me.

In your situation and assuming that you are not planning to constantly revisit a year of shots and play with them to make changes as an annual set, it may make sense to use your annual session approach for the year and at some point after the end of that year, when you feel your have probably done everything you want to do to those images, you can "archive" them into a catalogue. If keyworded etc., that will afford you the opportunity of searching all archived years for a given bird species or location (except the current year) in one pass. I am assuming that such a facility may be useful to you - however you may never need to do that in which case it might not be worth bothering with taking a few minutes to update the catalogue with the work from the yearly session.

It's very easy with these things to to set out with an objective of "wanting" to be able to do absolutely anything one could possibly think of whilst actually "needing" to do far less.

Perhaps the most important question in that respect is whether you also use some other software that also need to access the images and would that be an influence on how you wanted to store the original files.